This invention relates to a method of separating vinylcyclohexene and a butadiene extracting solvent from a mixture thereof. In particular, it relates to cooling the mixture to a temperature below 0.degree. C. which results in the formation of a vinylcyclohexene phase and an extracting solvent phase which are then physically separated.
In the manufacture of ethylene by cracking higher hydrocarbons, a sidestream of 4 carbon atom hydrocarbons is formed. This C.sub.4 sidestream is about 45 wt % butadiene with the remainder being butanes, butenes, and butynes. After the butynes have been hydrogenated to butanes and butenes the gaseous mixture is bubbled into a column of extracting solvent, where the butadiene is extracted, thereby separating it from the other gases. The extracting solvent is then heated to drive off the butadiene which is purified by distillation.
Some of the butadiene in the extracting solvent dimerizes to form 4-vinylcyclohexene (VCH). VCH reduces the selectivity of extracting solvent for butadiene which results in the dissolution of more impurities into the extracting solvent. At the present time this problem is handled by continuously pulling out a 5 wt % slipstream of the extracting solvent after the butadiene has been removed. The slipstream is distilled to separate the VCH from the extracting solvent. The overhead from the distillation column is called "butadiene dimer oil" (BDO), a multi-component azeotrope which has a typical composition of about 50 wt % VCH, about 40 wt % extracting solvent and about 10 wt % hydrocarbon impurities. BDO can constitute a significant portion of the liquid waste of some ethylene manufacturing plants. There is at present no economical method of separating the VCH from the extracting solvent in the BDO. Since BDO has no utility for chemical processing it is simply burned as a fuel.